Integrated circuits exist that are configured to receive and/or transmit radio frequency (RF) signals. More particularly, there are such integrated circuits that are configured to receive and/or transmit RF signals in the FM audio broadcast band. Further, some FM broadcast integrated circuits are included within other electronic devices. For reception of FM signals, it is often desirable to have an antenna to help receive those FM signals.
For an in-car portable electronic system (e.g., personal navigation device (PND)) with an FM tuner, the car lighter adapter (CLA) ground signal line has been previously used as an antenna for the FM tuner. However, where this is done, an inductive component is typically required to isolate the CLA ground from the portable electronic system ground, which is also the FM tuner ground. Because there is significant amount of current flowing through the CLA ground, it is inevitable that the isolation inductor must be large and bulky in size. This size problem poses significant design constraint in adopting the CLA ground as the FM antenna.
FIG. 2 (Prior Art) is a circuit diagram 200 where the ground signal line for a CLA is used as an antenna for an FM tuner that is part of a device, such as a PND. The CLA includes a connector 204 that is configured to be inserted into a lighter within a vehicle. The CLA also includes a power signal line (CLAPWR) 222 and a ground signal line (CLAGND) 220 that are coupled to a device power supply 202. The device power supply 202 then provides a supply voltage 208 and a ground 210 to the device. The supply 208 is used to power the device and to provide a supply voltage (Vcc) to the FM tuner integrated circuit (IC) 206. The ground 210 is used to provide the device ground 212 and the FM tuner ground 207. The RF input (RF IN) 205 for the FM tuner IC 206 is coupled to the CLA ground (CLAGND) signal line 220 through capacitor (C1) 214. In addition, an inductor (L1) 216 is coupled between the connector 204 and the device power supply 202 in series along the CLA ground (CLAGND) signal line 220 above the node where the capacitor (C1) 214 connects to the CLA ground (CLAGND) signal line 220. A shunt inductor (L2) 218 is also coupled between the RF input (RF IN) 205 and the device ground 212. As indicated above, for this embodiment, the inductor (L1) 216 is typically required to be relatively large and is undesirable in a portable system.